1. Field of the Invention
This invention relates to a programmable controller capable of executing a functional instruction formed by a ladder.
2. Description of the Related Art
A programmable controller (hereinafter referred to as a "PC") implements sequence control of a numerical control NC machine tool by means of software. The PC is adapted so that a sequence expressed by a ladder diagram is written in a program stored in memory means such as a ROM, and the program is called whenever needed in order to control a machine tool. Control instructions forming each step of sequence control include a basic instruction and a functional instruction. The former is an instruction implementing a one-bit logical operation such as AND or OR, and the latter is an instruction prepared in order to instruct a complicated function peculiar to a machine.
More specifically, since forming arithmetic functions necessary for various kinds of control using one-bit basic instructions requires too much actual labor, generally a repeatedly used control function is registered beforehand as a functional instruction in a subroutine and is called from a main program.
Recently, the greater diversification and sophistication of objects subjected to control has been accompanied by a need to execute various subroutines besides the functional instructions registered by ladder language. In such cases, there is a tendency on the user side to require higher speed sequence control. However, in the preparation of such a prior-art ladder diagram, programming must be performed using basic instructions and already registered functional instructions when a subroutine other than a registered functional instruction is executed. Consequently, there is an increase in the number of steps. Accordingly, in cases where an individual functional instruction corresponding to a machine tool is used repeatedly in preparing a ladder, there is a need for a system in which any desired functional instruction can be created on the user side.